Posterior process dynamic spacer

ABSTRACT

An interspinous spacer having a memory metal extension.

CONTINUING DATA

This divisional application claims priority from co-pending U.S. Ser.No. 10/796,359, filed Mar. 9, 2006, entitled Posterior Process DynamicSpacer, (Hawkins), the specification of which is incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

The leading cause of lower back pain arises from rupture or degenerationof lumbar intervertebral discs. Pain in the lower extremities is causedby the compression of spinal nerve roots by a bulging disc, while lowerback pain is caused by collapse of the disc and by the adverse effectsof articulation weight through a damaged, unstable vertebral joint.

In some cases, when a patient having a collapsed disc moves in extension(e.g., leans backward), the posterior portion of the annulus fibrosusmay further compress and extend into the spinal canal. This condition(called “spinal stenosis”) produces a narrowing of the spinal canal andimpingement of tissue upon the spinal cord, thereby producing pain.

There have been numerous attempts to provide relief for theseafflictions by providing a spacer that inserts between adjacent spinousprocesses present in the posterior portion of the spinal column. Ingeneral, these interspinous implants are adapted to allow flexion,rotation, translation and lateral bending movement in the patient, butresist or limit extension.

U.S. Pat. No. 6,068,630 (“Zuchermann”) discloses a spinal distractionimplant that alleviates pain associated with spinal stenosis byexpanding the volume in the spinal canal or neural foramen. Zuchermandiscloses a plurality of implants having a body portion and lateralwings. The body portion is adapted to seat between the adjacent spinousprocesses, while the wings are adapted to prevent lateral movement ofthe body portion, thereby holding it in place between the adjacentspinous processes. The designs disclosed in FIGS. 15, 80 and 84 ofZuchermann comprise central body having an integral wing.

Although the Zucherman device achieves spinal distraction, itnonetheless possesses some limitations. First, it is a multi-piecedesign, and so is subject to wear and implantation complexity. Second,since the Zuchermann central bodies have at least one integral wing, theclinician may encounter difficulty in sizing the central bodyindependently of delivering the lateral wings. Third, the expansivegeometry of the disclosed devices may not lend itself to minimallyinvasive surgical techniques seeking to conserve muscle mass and softtissue in the regions adjacent the spinous processes.

SUMMARY OF THE INVENTION

The present inventors have developed a number of flexible interspinousdevices having improvements over the conventional devices.

In a first embodiment, the extensions on one side of the implant aremade of a shape memory metal. This implant is inserted into theinterspinous space in a collapsed, low temperature form. When theimplant rises to the temperature of the patient's body, the upper andlower extensions made of memory metal transform to the austenitic phaseto extend upwards and downwards respectfully, thereby bracketing theupper and lower spinous processes and locking the implant in place.

In addition, since the shape memory extensions can deform elastically intheir austenitic phase, the ends of the extensions on a lateral side ofthe implant can be forced together, inserted through the interspinousspace, and then released, thereby allowing the extensions to spring backto their unconstrained shape.

Therefore, in accordance with the present invention, there is providedan interspinous implant for insertion into an interspinous space betweenadjacent spinous processes, comprising:

-   -   a) a central body having an upper surface for bearing against an        upper spinous process, a lower surface for bearing against a        lower spinous process, and first and second side portions,    -   b) a first upper extension extending upward from the first side        portions,    -   c) a second upper extension extending upward from the second        side portion, the upper extensions collectively defining an        upper bracket, and    -   d) a first lower extension extending downward from the first        side portion,    -   e) a second lower extension extending downward from the second        side portion, the lower extensions collectively defining a lower        bracket,

wherein each of the first upper and first lower extension comprises ashape memory metal.

In a second embodiment, the implant has bases fastened to opposite sidesof the same spinous process, and the bases are connected by a flexiblecord. The cord is adapted to have a flexibility and resiliency suchthat, during extension (when the spinous processes move closer towardsone another, the flexible cord provides a soft stop for the movement ofthe opposite spinous process, thereby gently limiting excessiveextension.

Therefore, in accordance with the present invention, there is providedan interspinous implant for insertion into an interspinous space betweena first and second spinous process, the first spinous process having afirst and second side, the implant comprising:

-   -   a) a first base having a side surface adapted for fixation to a        first side of the first spinous process,    -   b) a second base having a side surface adapted for fixation to a        second side of the first spinous process,        a first flexible ligament having a first end connected to the        first base and a second end connected to the second base.

In a third embodiment, the implant is a three-piece device having acentral body and a pair of lateral extensions, wherein the extensionsare slid through axial slots in the central body. Because neitherextension is integrally formed to the central body, the physician canfirst view and assess the placement of the central body prior to addingthe extensions without being shielded by the extension.

Therefore, in accordance with the present invention, there is providedan interspinous implant for insertion into an interspinous space betweena first and second spinous process, the implant comprising:

-   -   a) a central body having:        -   i. an upper surface for bearing against an upper spinous            process,        -   ii. a lower surface for bearing against a lower spinous            process,        -   iii. first and second side surfaces, and        -   iv. first and second axial through-holes, each through-hole            extending from the upper surface to the lower surface,    -   b) a first extension having an upper end and a lower end, the        first extension extending through the first axial through-hole        of the central body,    -   c) a second extension having an upper end and a lower end, the        second extension extending through the second axial through-hole        of the central body,        wherein the upper ends of the extensions collectively define an        upper bracket, and

wherein the lower ends of the extensions collectively define a lowerbracket.

In a fourth embodiment, the implant is a three-piece device having acentral body and a pair of lateral extensions, wherein side surfaces ofthe central body are connected to the extensions. As with the thirdembodiment, because neither extension is integrally formed to thecentral body, the physician can first view and assess the placement ofthe central body prior to adding the extensions without being shieldedby the extension.

Therefore, in accordance with the present invention, there is providedan interspinous implant for insertion into an interspinous space betweena first and second spinous process, the implant comprising:

-   -   a) a central body having:        -   i. an upper surface for bearing against an upper spinous            process,        -   ii. a lower surface for bearing against a lower spinous            process,        -   iii. first and second side surfaces defining a transverse            axis, and        -   iv. a first opening extending from the first side surface            into the body,    -   b) a first extension having an upper end, a lower end, an inner        surface, the first extension being separate from the central        body,    -   c) a second extension having an upper end, a lower end, an inner        surface, the second extension being separate from the central        body,        wherein the first side surface of the central body contacts the        inner surface of the first extension,        wherein the second side surface of the central body contacts the        inner surface of the second extension,        wherein the upper ends of the extensions collectively define an        upper bracket, and        wherein the lower ends of the extensions collectively define a        lower bracket.

In a fifth embodiment, the interspinous implant has a pair of U-shapedhooks adapted to cradle the opposing processes and a connection piecetherebetween. The hooks have leading and trailing ends and are furtheradapted to be slid laterally around the spinous process.

Therefore, in accordance with the present invention, there is providedan interspinous implant for insertion into an interspinous space betweena first and second spinous process, the implant comprising:

-   -   a) an upper hook having a leading end, a trailing end, an upper        bearing surface adapted to bear against the first spinous        process, and a lower surface,    -   b) a lower hook having a leading end, a trailing end, and a        lower bearing surface adapted to bear against the first spinous        process, and an upper surface,    -   c) a central body having:        -   i. an upper surface adapted for connection to the lower            surface of the upper hook, and        -   ii. a lower surface adapted for connection to the upper            surface of the lower hook.

DESCRIPTION OF THE FIGURES

FIGS. 1 a and 1 b disclose embodiments of the same memory metal implantof the present invention in the austentitic and martensitic phases.

FIG. 2 discloses the memory metal implant of FIG. 1 after beingimplanted in the interspinous space and returning to its austeniticform.

FIGS. 3 a and 3 b disclose a second embodiment of the present inventionhaving two flexible cords between two bases.

FIG. 3 c discloses the implant of FIGS. 3 a-b implanted so as tolaterally span an interspinous space to provide a soft stop for aspinous process during extension.

FIGS. 4 a and 4 b disclose components of a third embodiment of thepresent invention wherein the central body has axial openings adaptedfor the reception of extensions.

FIG. 4 c shows the assembly of the components of FIGS. 4 a and 4 b.

FIGS. 5 a-5 d disclose a fourth embodiment of the present inventionwherein the central body and extensions are connected by rivets.

FIGS. 6 a-6 c disclose a fifth embodiment of the present inventionhaving a pair of U-shaped hooks.

FIG. 7 is a side view of a functional spinal unit of the human anatomy.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention, the term “interspinous”refers to the volume located between two adjacent spinous processes ofadjacent vertebrae. The terms “anterior” and “posterior” are used asthey are normally used in spinal anatomy. Accordingly, the “anterior”portion of the interspinous device is that portion rests relativelyclose to the spinal cord, while the “posterior” portion of theinterspinous device is that portion rests relatively close to the skinon the patient's back. Now referring to FIG. 7, there is provided ananatomic “functional spinal unit” or FSU comprising an upper vertebraehaving an upper vertebral body V_(U) and an upper spinous process SPu, alower vertebra having a lower vertebral body V_(L) having a lowerspinous process SP_(L). The vertebral bodies lies in the anterior Aportion of the FSU, while the spinous processes lie in the posteriorportion P of the FSU. Disposed between the vertebral bodies is a discspace DISC. Disposed between the spinous process is an “interspinousregion”. Disposed between the spinous process and the vertebral body ofeach vertebra is a lamina L. The supraspinous ligament SSL liesposterior to the spinous processes. The posterior longitudinal ligamentPLL lies posterior to the vertebral bodies.

Now referring to FIG. 1 a, there is provided an interspinous implant 1for insertion into an interspinous space between adjacent spinousprocesses, comprising:

-   -   a) a central body 5 having an upper surface 7 for bearing        against an upper spinous process, a lower surface 9 for bearing        against a lower spinous process, and first 11 and second 13 side        portions,    -   b) a first upper extension 15 extending upward from the first        side portion,    -   c) a second upper extension 17 extending upward from the second        side portion, the upper extensions collectively defining an        upper bracket, and    -   d) a first lower extension 19 extending downward from the first        side portion,    -   e) a second lower extension 21 extending downward from the        second side portion, the lower extensions collectively defining        a lower bracket,        wherein each of the second upper and second lower extensions        comprises a shape memory metal.

In some preferred embodiments, the first upper and first lowerextensions are adapted to extend upwards and downwards in an austeniticphase, and laterally in the martensitic phase. This implant is insertedinto the interspinous space in a collapsed, low temperature(martensitic) form, as shown in FIG. 1 b. Now referring to FIG. 1 a,when the implant rises to the temperature of the patient's body, theupper and lower extensions made of memory metal transform to theaustenitic phase to extend upwards and downwards respectfully, therebybracketing the upper and lower spinous processes and locking the implantin place.

The austenitic and martensitic forms of the implant are respectivelyshown in FIGS. 1 a and 1 b. The device as implanted is shown in FIG. 2.

Because the memory-metal induced transformation of each of the secondupper and second lower extensions occur in response to a change intemperature, the desired shape changes occur without any action from thesurgeon. Accordingly, the implantation of this device is very simple.

In one preferred embodiment, the side surface of the central body fromwhich the memory metal extensions extend has a slight recess. Thisrecess reduces the stress produced by the transformation.

In some embodiments, the implant is a unitary body. The unitary natureof the body provides for ease of manufacturing and implantation, andreduces the stress on the implant.

In some embodiments, the first upper and first lower extensions areadapted to superelastically extend sideways in a martensitic phase. Thisembodiment provides for a reduced stress upon the implant.

In some embodiments, at least one of the memory metal extensions has achamfered end 23. The chamfer increases the ease of insertion on theseextensions into the interspinous space. In preferred embodiments, eachof the first upper and first lower extensions has a chamfered end.

In some embodiments, the upper and lower surfaces of the central bodydefine a body height H_(CB), wherein each of the first upper and firstlower extensions have an end defining an extension height therebetweenH_(E), and wherein the extension height H_(E) is less than the centralbody height H_(CB). When the extension height H_(E) is less than thecentral body height H_(CB), the implant may more easily be implantedinto the interspinous space. Preferably, each end of the second upperand second lower extensions contact one another in the martensiticphase.

Preferably, the shape memory material is a nickel-titanium alloy.

Now referring to FIGS. 3 a and 3 b, there is provided an interspinousimplant 31 for insertion into an interspinous space between a first andsecond spinous process, the first spinous process having a first andsecond side, the implant comprising:

-   -   a) a first base 33 having a side surface 35 adapted for fixation        to a first side of the first spinous process,    -   b) a second base 37 having a side surface 39 adapted for        fixation to a second side of the first spinous process, and    -   c) a first flexible ligament 41 having a first end 43 connected        to the first base and a second end 45 connected to the second        base.

Now referring to FIG. 3 a, in this embodiment, the implant has basesadapted to fasten to opposite sides of the same spinous process, and thebases are connected by a flexible cord. The surgeon simply takes theimplant in its open form (as in FIG. 3 b), inserts the leading base ofthe implant laterally into a first side of the interspinous space, andthen pulls the leading end as it emerges from the second side of theinterspinous space. The surgeon then folds the implant so that each baseabuts its respective side of the lower spinous process. Now referring toFIG. 3 c, the surgeon then adjusts the position of the device so thatits apex 46 of the ligament is at a position between the spinousprocesses that will provide the appropriate amount of distraction forthe patient's relief of pain. The surgeon then fastens the bases to thelower spinous process.

The cord has flexibility and resiliency such that, during extension(when the spinous processes move closer towards one another, theflexible cord provides a soft stop for the movement of the oppositespinous process, thereby gently limiting excessive extension. Since thelimitation on extension is provided gradually and gently (i.e., it isnot a hard stop), it is believed that there will be less wear of therespective contacting surfaces, thereby prolonging the life of theimplant.

Referring to FIG. 3 a, in some embodiments, each of the first and secondbases comprises an upper surface 47,48, wherein the first end of thefirst flexible ligament is connected to the upper surface of the firstbase, and the second end of the first flexible ligament is connected tothe upper surface of the second base. When the ligament is connected tothe upper surfaces of bases (as opposed to the side surfaces), thelength of and stresses upon the ligaments are minimized.

In some embodiments, the upper surfaces of each base form an angle of nomore than 180 degrees, preferably less than 180 degrees, more preferablybetween 100 degrees and less than 180 degrees. In this range, theligament takes on an arcuate shape well suited to flexibly accept andresist extension of the upper spinous process.

In some embodiments, the implant further comprises a second flexibleligament 49 having a first end 51 connected to the first base and asecond end 52 connected to the second base. The provision of the secondflexible ligament is advantageous because the spinous processes have aproportionally larger dimension from the anterior to the posterior(thereby causing a posterior narrowing of the interspinous space). Inaddition, the provision of a second ligament distributes compressiveextension loads more evenly along the processes.

In some embodiments, each base comprises a transverse hole 50 throughpassing through the side surface adapted for fixation to a side of aspinous process. The tranverse holes allows the surgeon to pass afixation device (such as a screw) through each hole, thereby fixing theimplant to the lower spinous process.

In some embodiments, the first ligament is made of a flexible polymer,and is preferably selected from the group consisting of polyester(preferably, Dacron®) and polyethylene. Preferably, the ligament is alongitudinal element having a thickness of between 3 cm and 8 cm. Theselection of a thickness in this range, along with the selection of aflexible polymer as the material of construction, should provide aligament that is suitably flexible to provide a gentle stop to extremeextension.

In other embodiments, the ligament takes the form of a fabric or strap.When the fabric embodiment is selected, it is desirable to use only asingle ligament.

In some embodiments, the bases are made of a material selected from thegroup consisting of ultra high molecular weight polyethylene and PEEK.These materials are well known biocompatible materials of constructionfor load bearing medical devices.

Now referring to FIGS. 4 a-c, there is provided an interspinous implant51 for insertion into an interspinous space between a first and secondspinous process, the implant comprising:

-   -   a) a central body 53 having:        -   i. an upper surface 55 for bearing against an upper spinous            process,        -   ii. a lower surface 57 for bearing against a lower spinous            process,        -   iii. first 59 and second 61 side surfaces, and        -   iv. first 63 and second 65 axial through-holes, each            through-hole extending from the upper surface to the lower            surface,    -   b) a first extension 67 having an upper end 69 and a lower end        71, the first extension extending through the first axial        through-hole of the central body,    -   c) a second extension 73 having an upper end 75 and a lower end        77, the second extension extending through the second axial        through-hole of the central body,        wherein the upper ends of the extensions collectively define an        upper bracket, and wherein the lower ends of the extensions        collectively define a lower bracket.

In use, the surgeon first orients the central body portion of theimplant so that its throughholes run in the (axial) saggital plane. Thesurgeon then inserts the oriented central body laterally into theinterspinous space so that one axial throughhole is disposed on one sideof the interspinous space and the second axial throughhole is disposedon the second side of the interspinous space. The surgeon then adjuststhe position of the device so that it is approximately centered abouteach spinous process. The surgeon then inserts the extensions into therespective axial throughholes to secure the implant to the spinousprocesses.

Because neither extension is connected to the central body duringinsertion, but rather may be inserted separately, the physician canfirst view and assess the placement of the central body prior to addingthe extensions without being visually shielded by the extension. Inaddition, the separate insertion of the extensions lowers the lateralspan of the implant during insertion, thereby causing less damage to thesensitive musculature surrounding the interspinous space.

In addition, since the human spinous process is often a source ofsignificant interindividual variation, each of the central body andextensions may be provided in different shapes and sizes, so that thesurgeon can intra-operatively select the appropriate central body andextensions, thereby providing greater surface area contact between theimplant and the adjacent processes and minimizing stresses. Differentshapes (which, in some cases, have very small lateral profiles) may besuitable for different anatomical interspinous spaces. In addition, thecentral body may be provided in different heights so that the surgeoncan select the central body producing the most appropriate degree ofinterspinous space distraction.

In some embodiments, the central body has a saggital profile comprisinga substantially parallel anterior portion and an inwardly taperingposterior portion. It is believed that this profile more closelyresembles the profile of the interspinous space, and so should providefor more contact therebetween and reduced stresses.

Since the physiologic loads experienced by central body during extensionare relatively low (e.g., only about 20 pounds-force), the central bodymay be made of materials such as UWMWPE or PEEK. These materials arealso preferred for the suitability in medical imaging procedures.

In some embodiments, the extensions comprise a centrally located recess79 having a length substantially similar to the height of the centralbody. The recess allows the extension to snap into place when it isappropriately situated within the central body, thereby insuring asecure fit. When the extension is made of a metal material (such as atitanium alloy), the extension may further desirably comprise aninternal slot (not shown) adapted to behave in a spring-like mannerduring extension insertion, thereby facilitating the insertion of theextension.

Now referring to FIG. 5 a, there is provided an interspinous implant 101for insertion into an interspinous space between a first and secondspinous process, the implant comprising:

-   -   a) a central body 103 having:        -   i. an upper surface 105 for bearing against an upper spinous            process,        -   ii. a lower surface 107 for bearing against a lower spinous            process,        -   iii. first 109 and second 111 side surfaces defining a            transverse axis, and        -   iv. a transverse 113 through-hole extending from the first            side surface to the second side surface,    -   b) a first extension 121 having an upper end 123, a lower end        125, an inner surface 127, and a transverse throughhole 129, the        inner surface of the first extension contacting the first side        surface of the central body and aligning the transverse        througholes,    -   c) a second extension 131 having an upper end 133, a lower end        135, an inner surface 137, and a tranverse throughhole 139, the        inner surface of the second extension contacting the second side        surface of the central body and aligning the transverse        througholes, and    -   d) a rivet adapted to connect the extensions to the central        body.        wherein the upper ends of the extensions collectively define an        upper bracket, and        wherein the lower ends of the extensions collectively define a        lower bracket.

In use, the surgeon first inserts the central body laterally into theinterspinous space so that one opening of the tranverse throughhole isdisposed on one side of the interspinous space and the second opening ofthe throughhole is disposed on the second side of the interspinousspace. The surgeon then adjusts the position of the device so that it isapproximately centered about each spinous process, and orients thecentral body portion of the implant so that its throughhole runs in themedial-lateral plane.

Next, the surgeon selects the appropriate extensions and placesconnecting pins through the throughholes of the extensions.

In some embodiments, as in FIG. 5 b, the extensions are spaced from eachother a distance that is substantially greater than the width of thespinous process. Since the supraspinous ligament should provide a tightgrip upon the inserted central body, there should be no need forfastening the extensions to the spinous process. In this instance, theextensions merely serve as stops of excessive medial-lateral movement ofthe central body. Accordingly, providing a space between the sidesurfaces spinous processes and the inner surfaces of the extensionsshould minimize wear.

In some embodiments, the extensions have an inner surface 144 having aconvex contour and an outer surface 140 having a concave contour. Asshown in FIG. 5 b, this convex contour is preferably adapted to matchthe contour of the spinous process. This contour should minimize wear ofand stress upon the extension. The concave contour is preferably adaptedto match the erector spinae portion of the low back musculature.

In some embodiments, the extensions have an anterior surface 142 havinga concave contour 143. As shown in FIG. 5 c, this concave contour ispreferably adapted to match the convex contour of the lamina archportion of the vertebral body. This contour should minimize wear of andstress upon the extension, and be less invasive to the patient's softtissues.

Referring back to FIG. 5 a, the rivet may include any conventionalriveting assembly. In some embodiments, the rivet comprises:

-   -   i) a first connecting pin 141 adapted to fit in the first        transverse throughole and having a male end 143, and    -   ii) a second connecting pin 145 adapted to fit in the second        transverse throughole and having a female end 147.        In the embodiment shown in FIG. 5 a, the rivets are shown as        being separately constructed from the extensions. However, in        other embodiments, the rivets may be integral with the        extensions. Similarly, each side of the central body may be        separately riveted to its respective extension.

In some embodiments, as shown in FIG. 5 d, the rivet is located about inthe center of the extension. In other embodiments, as shown in FIG. 5 c,the rivet 149 is located in the bottom half of the extension. It isbelieved that locating the rivet in the bottom half of the extensionsdesirably provides a good match fit with the bony contours of thevertebral body.

The implants of FIGS. 5 a-d may be suitably manufactured from anysuitable biomaterial, including metals such as titanium alloys,chromium-cobalt alloys and stainless steel) and polymers (such as PEEK,carbon fiber-polymer composites and UHMWPE. In preferred embodiments,the central body is made of UHMWPE (to provide moderate stiffness) andthe extensions are made of a carbon fiber-PEEK composite (to providestiffness to the extensions).

Now referring to FIGS. 6 a-c, there is provided an interspinous implant151 for insertion into an interspinous space between a first and secondspinous process, the implant comprising:

-   -   a) an upper hook 153 having a leading end 155, a trailing end        157, an upper bearing surface 159 adapted to bear against the        first spinous process, and a lower surface,    -   b) a lower hook 163 having a leading end 165, a trailing end        167, and a lower bearing surface 169 adapted to bear against the        first spinous process, and an upper surface,    -   c) a central body 171 having:        -   i. an upper surface 173 adapted for connection to the lower            surface of the upper hook, and        -   ii. a lower surface 174 adapted for connection to the upper            surface of the lower hook.

Now referring to FIG. 6 a, the surgeon simply inserts a leading base ofthe upper hook laterally into a first side of the interspinous space,and then pulls the leading end laterally and upward as it emerges fromthe second side of the interspinous space. The surgeon then repeats thisprocess for the lower hook, so that each hooks envelops its respectiveside of the upper and lower spinous processes. The surgeon then insertsthe central body into the space between the hooks and connects each hookto the central body, thereby fixing the implant. FIG. 6 b shows theassembled implant.

In some embodiments, the leading and trailing ends of the upper hookextend in substantially a first same direction (more preferably,upward), and the leading and trailing ends of the lower hook extend insubstantially a second same direction (more preferably, downward). Inthis condition, the profile of the implant is relatively low.

In some embodiments, the upper surface of the central body is adaptedfor connection to the lower surface of the upper hook by a male-femaleconnection. In preferred embodiments, thereof the upper surface of thecentral body is adapted for connection to the lower surface of the upperhook by a dovetail connection 176. The dovetail connection is believedto produce a highly secure fixation.

In some embodiments, the upper surface of the central body has a femalerecess traversing the upper surface in a direction from the leading endto the trailing end. In others, the upper surface of the central bodyhas a projection 177 traversing the upper surface in a direction fromthe leading end to the trailing end. In each of these cases, theorientation of the mating feature allows fixation to occur in the samemotion as insertion of the central body. In preferred embodimentsthereof, the upper surface of the central body has a dovetail featuretraversing the upper surface in a direction from the leading end to thetrailing end.

We claim:
 1. An interspinous implant for insertion into an interspinousspace between adjacent spinous processes, comprising: a) a central bodyhaving an upper surface for bearing against an upper spinous process, alower surface for bearing against a lower spinous process, and first andsecond side portions, b) a first upper extension extending upward fromthe first side portions, c) a second upper extension extending upwardfrom the second side portion, the upper extensions collectively definingan upper bracket, and d) a first lower extension extending downward fromthe first side portion, e) a second lower extension extending downwardfrom the second side portion, the lower extensions collectively defininga lower bracket, wherein each of the first upper and first lowerextensions comprises a shape memory metal, wherein the implant is aunitary body, wherein the first upper and first lower extensions areadapted to superelastically extend sideways in a martensitic phase, andwherein the first upper and first lower extensions are adapted to extendupwards and downwards in an austenitic phase.
 2. The implant of claim 1wherein the first upper and first lower extensions are adapted tosuperelastically extend in a substantially parallel manner in amartensitic phase.
 3. The implant of claim 2 wherein at least one of thefirst upper and first lower extensions has a chamfered end.
 4. Theimplant of claim 2 wherein the upper and lower surfaces of the centralbody define a body height H_(CB), wherein each of the first and secondlower extensions have an end defining an extension height therebetweenH_(E), and wherein the extension height H_(E) is less than the centralbody height H_(CB).
 5. The implant of claim 2 wherein the first upperand first lower extensions are adapted to extend in a substantiallyparallel manner in an austenitic phase.
 6. The implant of claim 2wherein the upper and lower surfaces of the central body have ananterior-posterior groove adapted to cradle the spinous processes. 7.The implant of claim 2 wherein the upper and lower surfaces of thecentral body are parallel.
 8. The implant of claim 2 wherein the upperand lower surfaces of the central body are angled.